In many communication systems, an oscillator is employed to generate a reference oscillating signal from which other signals and/or clocks are produced. For example, the reference oscillating signal may be used to generate one or more clocks for driving digital and analog circuitry. Additionally, the reference oscillating signal may be employed in a local oscillator (LO) for downconverting radio frequency (RF), intermediate frequency (IF), or other signals to lower or baseband frequencies, and/or for upconverting baseband signals to IF, RF, or other higher frequencies.
Many of these communication systems are portable systems, such as cellular telephones, personal digital assistants (PDAs), handheld devices, and other portable communication devices. These portable communication systems typically rely on a limited power source, such as a battery, to perform the various intended operations. A limited power source typically has a continuous use lifetime that depends on the amount of power drawn by the portable device. It is generally desired to extend the continuous use lifetime as much as possible. Accordingly, portable communication systems are more frequently designed to consume less and less power.
With regard to oscillators, their power consumption generally depends on the frequency tuning range of the oscillating signal being generated. For example, if an oscillator is designed using a very expensive and precise crystal (Xtal), the frequency tuning range need not be that large. Consequently, the power consumption of the oscillator may be maintained relatively low. On the other hand, if an oscillator is designed using an inexpensive and not-so-precise Xtal, the frequency tuning range generally needs to be larger in order to ensure that the frequency of the oscillating signal is maintained within specification. Unfortunately, the power consumption of the oscillator is generally greater with the wider frequency tuning range. This is better explained with reference to the following example.
FIG. 1A illustrates a block/schematic diagram of a conventional apparatus 100 for generating an oscillating signal. The conventional apparatus 100 typically comprises a negative resistance circuit 102 and a resonator coupled in a feedback configuration with the negative resistance circuit. The resonator, in this example, includes a Xtal 104 and a pair of variable capacitors CP, typically referred to in the relevant art as Pierce capacitors. The variable capacitors CP serve to provide external tuning of the frequency of the oscillating signal generated by the apparatus 100.
FIG. 1B illustrates a graph of an impedance versus frequency of the resonator of the conventional apparatus 100. If, for example, the Pierce capacitors CP of the conventional oscillator 100 are removed, the frequency of the oscillating signal is dictated substantially by the Xtal 104. In such a case, the frequency of the oscillating signal falls substantially on the parallel resonance of the Xtal 104, as indicated in the graph. At the parallel resonance, the power drawn by the negative resistance circuit 102 is substantially minimized because the impedance of the resonator is substantially maximized.
If the Xtal 104 is very precise from lot-to-lot, the parallel resonance for the Xtal does not significantly change from lot-to-lot. If the tolerance of the parallel resonance is within the frequency deviation specification of the oscillator 100, then the Pierce capacitors CP need not be required, and the oscillator may be operated in a power efficient manner. However, such a precise Xtal 104 is typically very expensive. Thus, in order to reduce cost of the oscillator 100, a less precise and cheaper Xtal 104 may be employed. This may have the adverse consequence of the parallel resonance of the Xtal 104 from lot-to-lot varying more than the frequency deviation specification of the oscillator 100. In such a case, frequency tuning may be required, thereby, mandating the use of one or more variable capacitors CP.
The effects of adding capacitance CP to the resonator of the oscillator 100 is to decrease the frequency of the oscillating signal. This has the consequence of lowering the impedance of the resonator from the impedance associated with the parallel resonance of the Xtal 104 towards the impedance associated with the series resonance of the Xtal, as indicated by the oscillator tunable range in the graph. As the impedance of the resonator moves closer to the series resonance of the Xtal 104, the negative resistance circuit 102 consumes more power.
The power consumption is related to the pullability or frequency difference between the parallel resonance and the series resonance of the Xtal 104. For a given frequency of the oscillating signal, the power consumption of the oscillator 100 is less if the pullability is larger. Or, conversely, for a given frequency of the oscillating signal, the power consumption of the oscillator 100 is more if the pullability is smaller. Thus, there is a need to increase the pullability of the oscillator 100 in order to operate the oscillator 100 in a more power efficient manner.